Various types of gas compressors have been proposed heretofore (e.g., Patent Literature 1).
FIG. 6 shows a compression block used in a conventional gas compressor.
This compression block (block part) has a tubular cylinder block 100 and paired side blocks 101 placed on the left and right ends of the cylinder block 100 to sandwich the cylinder block 100. The cylinder block 100 and the paired side blocks 101 define a cylinder chamber 104 within the compression block. The cylinder block 100 is provided with an intake port 110 and two discharge ports 108.
A rotor 102 is rotatably housed in the cylinder chamber 104. Multiple vane grooves 106 are formed in an outer circumferential surface of the rotor 102 at intervals in a circumferential direction (rotary direction W) of the rotor 102. Vanes 103 (103a, 103b, 103c) are placed in the respective vane grooves 106 such that the vanes 103 can emerge from the outer circumferential surface of the rotor 102. In the vane grooves 106, backpressure spaces 107 (107A, 107B, 107C) are formed behind the vanes 103. Each of these backpressure spaces 107 opens onto both left and right end surfaces of the rotor 102.
An intermediate-pressure supply groove (intermediate-pressure supply part) 113 and a high-pressure supply groove (high-pressure supply part) 114 are formed in an end surface of each of the side blocks 101 on the cylinder chamber 104 side (inner end surface), at positions on a rotational trajectory of the backpressure spaces 107. The intermediate-pressure supply groove 113 is supplied with fluid (e.g., oil) at an intermediate pressure which is higher than the pressure of refrigerant gas taken into compression chambers 105 and lower than the pressure of refrigerant gas discharged from the compression chambers 105. The high-pressure supply groove 114 is supplied with fluid at a high pressure which is equivalent to the pressure of refrigerant gas discharged from the compression chambers 105.
In the cylinder chamber 104, the compression chamber 105 (105a, 105b, 105c) is defined by an inner circumferential surface of the cylinder chamber 104, the outer circumferential surface of the rotor 102, and corresponding two vanes 103 adjacent in the circumferential direction of the rotor 102. While the rotor 102 rotates, an intake cycle, a compression cycle, and a discharge cycle are repeatedly carried out in each compression chamber 105.
In the intake cycle in each compression chamber 105, the volume of the compression chamber 105 increases gradually as the rotor 102 rotates, and the refrigerant gas is taken into the compression chamber 105 through the intake port 110.
In the compression cycle in the compression chamber 105, the volume of the compression chamber 105 decreases gradually as the rotor 102 rotates, and the refrigerant gas in the compression chamber 105 is compressed.
In the discharge cycle in the compression chamber 105, the volume of the compression chamber 105 decreases gradually as the rotor 102 rotates, and when the pressure of the refrigerant gas (refrigerant pressure) inside the compression chamber 105 reaches a predetermined pressure, an on-off valve 109 opens to discharge the refrigerant gas from the compression chamber 105 through the discharge port 108.
In such a series of cycles, the vanes 103a, 103b, 103c receive the pressure of the refrigerant gas in the corresponding compression chambers 105a, 105b, 105c, the pressure acting in directions in which the vanes 103a, 103b, 103c retreat into their corresponding vane grooves 106 (referred to as “retreating directions” below). Meanwhile, the pressure of the fluid in the backpressure spaces 107 (backpressure) acting on the vanes 103a, 103b, 103c presses the tips of the vanes 103a, 103b, 103c against the inner circumferential surface of the cylinder chamber 104. This backpressure enables the vanes 103 to restrict flow of the refrigerant gas between the compression chambers 105 adjacent in the circumferential direction of the rotor 102, ensuring compression of the refrigerant gas in each compression chamber 105a, 105b, 105c. 
The pressure of the refrigerant gas in each compression chamber 105 acting on the vane 103 in the retreating direction is relatively low in the intake cycle and in the early compression cycle. Thus, in areas corresponding to these cycles, the backpressure space 107 is caused to communicate with the intermediate-pressure supply groove 113 so that intermediate pressure of the fluid in the intermediate-pressure supply groove 113 may act on the vane 103 as backpressure. On the other hand, the pressure of the refrigerant gas in the compression chamber 105 acting on the vane 103 in the retreating direction is relatively high in the late compression cycle and the discharge cycle. Thus, in the area corresponding to these cycles, the backpressure space 107 is caused to communicate with the high-pressure supply groove 114 so that high pressure of the fluid in the high-pressure supply groove 114 may act on the vane 103 as backpressure. The backpressure acting on the vanes 103 is thus changed according to the pressure of the refrigerant gas in the compression chambers 105 acting on the vanes 103 in their retreating directions, so that the vanes 103 slide on the inner circumferential surface of the cylinder chamber 104 with a minimum resistance to save fuel consumption.